1. Field of the Invention
This invention relates in general to range finding by light beam, light detector and time delay, such as may be used in field and land surveying. An improved precision may be thus achieved while at the same time natural, non cooperative objects may be measured, although the invention is not to be limited to these particular applications.
2. Description of the Prior Art
Electrooptical distance meters are well known, for example from U.S. Pat. No. 4,274,736 and from Allgemeine Vermessungsnachrichten, Karlsruhe, FR Germany, 6/1973, page 201-207. In that case the light beam is produced continuously by an infrared source which is amplitude modulated at a frequency of 15 MHz. The measured time delay is the phase difference between the modulation as transmitted and as received. The phase difference is measured digitally by means of electronic circuits after a frequency conversion. Measurement accuracy is achieved simply by counting a great number (e.g. 300,000) of phase differences from consecutive modulation periods and by computing the distance by the arithmetic mean of the phase differences.
Erroneous results caused by interference of the reflected light beam or by altogether missing the reflector may be avoided by a simple blanking circuit which interrupts signal evaluation at low level of reflected beam intensity. This may be done because such apparatus is used exclusively with reflectors at the other end of the distance which produce a rather strong reflected beam.
There are also known laser distance meters which transmit light impulses by a laser generator (e.g. DE-OS 28 40 605). In that case reflectors may be unnecessary, but there is a greater risk of errors due to troubled reflections, misaiming, multiple echos, etc., than with the first mentioned systems. These risks are reduced in a known manner by adjustable blanking circuits which enable reception only during a time window corresponding to an adjustable reduced distance range. These blanking circuits are principally designed to protect the light receiver from damage by exposure to bright illumination, blanking of false echos is only a secondary effect.
Another known electrooptical range finder (DE-AS 2 420 194) with evaluation of a number of consecutively measured phase differences has a digital circuit for computing the statistical variance of these measured values. The variance is compared to an internally stored limit and the range result is rejected if the variance exceeds the limit. Thus no selection is made between good and faulty single measured values. Even acceptable range results may be impaired by erratic reflexes without any information on the measured distance. An object of the present invention is to provide a method and an apparatus for electrooptical distance measurement with an improved accuracy for non-cooperative objects as well as for objects with reflectors. Another object is to reject from evaluation any single measured values with a certain probability of being in error.
The foregoing objects, other objects as well as numerous specific features of the present invention are set forth in the following disclosure.